To burn a given quantity of fuel, a given amount of air is theoretically required. This perfect match is called stoichiometric combustion. In practice, however, excess air is required in order to burn all of the combustibles within a limited space, such as the confines of the furnace of a steam generator. The use of excess air presents some problems, particularly in coal-fired furnaces where the fuel is introduced tangentially of an imaginary circle within the furnace. The excess air makes oxygen available for the formation of NO.sub.x, which is undesirable. NO.sub.x forms most readily at high temperatures, and thus if the excess air is introduced as overfire air (OFA), above the burner level within the furnace, the formation of NO.sub.x is minimized. In tangential firing there is a large number of flow patterns which, because of the rotating "fireball", deposit solids onto the furnace sidewalls. In addition, a temperature imbalance in the exhaust gases leaving the furnace can exist, because of the spinning flow effect caused by the tangential firing of the fuel.
Patent application No. 474,114 entitled "System For Injecting Overfire Air Into A Tangentially-Fired Furnace" and filed on Mar. 10, 1983 addressed some of the above problems. That application suggests introducing OFA into a tangentially fired furnace, with the OFA being introduced with a swirling direction opposite to that of the fuel, so that the "spin" is cancelled out, thus resulting in straight or "plug" flow of the gases leaving the furnace.